The present invention relates to the hydrometallurgical recovery of precious metal values from refractory precious metal ores containing preg-robbing carbonaceous material.
Conventionally, precious metals have been extracted from ore materials by lixiviation or leaching with cyanide-containing solutions. It has been found however that some gold ores do not respond well to conventional lixiviation because of the presence of impurities that interfere with the leaching process. These ores are termed "refractory".
One common cause of the refractoriness of gold ores is organic carbonaceous matter that is associated with some deposits. This carbonaceous matter is believed to adsorb solubilized gold complexes from lixiviant solutions back into the ore. The adsorbed gold is not recovered, and remains with the ore material and is eventually carried off with the tailings, leading to poor gold recovery. This can be a very serious problem, as a small amount of carbonaceous matter can adsorb essentially all of the solubilized gold in an entire cyanide lixiviation circuit. This is sometimes referred to as poisoning the circuit. In other cases, the carbonaceous matter is believed to coat the gold, and thereby prevent the lixiviant solution from gaining access to it. In other words, carbon can "rob" precious metal values, and gold in particular, from the lixiviant solution that is "pregnant" therewith. This characteristic is referred to as "preg-robbing."
It is believed that the carbonaceous content that participates in preg-robbing comprises an activated carbon-type carbon material, long-chain hydrocarbons and organic acids, such as humic acid. See Sibrell, P. L. et al., Spectroscopic analysis of Passivation Reactions for Carbonaceous Matter from Carlin Trend Ores, GOLD 90 PROCESS MINERALOGY X, pp. 355-363 (1990). Adsorption of the gold lixiviant complex by carbonaceous material is very complicated, due to three major factors. First, the precise chemical and physical nature of the carbonaceous matter is difficult to define, and varies from one ore body to the next. Second, the mechanism by which the carbonaceous material adsorbs gold is still being investigated. Third, although it has been known for some time that preg-robbing carbonaceous material can be passivated, or treated so as not to adsorb gold, the mechanism by which this occurs is not fully understood.
Many procedures have been investigated in an effort to passivate or deactivate the preg-robbing potential of carbonaceous ores, but none have been entirely satisfactory when applied to low grade refractory ores. The procedures heretofore tried include roasting, kerosene pretreatment, flotation, aqueous chlorination, chemical oxidation and biological deactivation. The preferred approach for the recovery of previous metal values from preg-robbing carbonaceous ore materials has been to deactivate or remove the preg-robbing components in the ore material using one of the aforereferenced pretreatment techniques followed by lixiviation with cyanide-solution. Examples of such attempts can be found in U.S. Pat. No. 5,127,942 to Brierley et al. which describes the deactivation the preg-robbing carbonaceous component in refractory ores using a specific microbial consortium, followed by recovery of precious metal from the carbon-deactivated residue by cyanidation; U.S. Pat. No. 4,801,329 to Clough et al. which describes the use of a chemical oxidation pretreatment to enable a precious metal to be extracted from carbonaceous ores preferably by cyanidation. The deactivation of the preg-robbing carbonaceous components with chemical agents, such as taught by Clough, and with biological/biochemical agents, such as taught by Brierley, introduces additional expense and complexity to the processing of refractory ores materials.
Heretofore thiosulfate lixiviant has been suggested for recovering precious metals from difficult to treat ores. U.S. Pat. No. 4,654,078 to Perez et al. describes the use of copper-ammonium thiosulfate to recover precious metals from difficult-to-treat ores, especially those containing manganese and/or copper. The presence of copper and/or manganese contraindicates the use of cyanide solution leaching because such materials increase cyanide consumption. U.S. Pat. Nos. 4,369,061 and 4,269,622 to Kerley describe lixiviating with an ammonium thiosulfate leach solution containing copper to recover precious metals from difficult-to-treat ores, particularly those containing copper, arsenic, antimony, selenium, tellurium and/or manganese, and most particularly those containing manganese. U.S. Pat. No. 4,070,182 to Genik-Sas-Berezlosky et al. describes the recovery of gold from copper-bearing sulfidic material containing gold using a secondary leach with ammonium thiosulfate.
Nothing in the prior art has suggested that excellent precious metal recovery yields could be achieved from preg-robbing carbonaceous ores material, including low grade materials, without a pretreatment step to deactivate or remove the preg-robbing components in the ore material by using thiosulfate lixiviation under controlled conditions.
Despite the growing world-wide interest in recovering precious metals from carbonaceous ores, and substantial work which has been done to develop a viable technology for doing so, a fully satisfactory process for metal recovery from most carbonaceous ore materials has yet to be provided.
Therefore, it is an object of the invention to provide a process for recovering at least one precious metal from preg-robbing carbonaceous ore, without the necessity of first subjecting the preg-robbing carbonaceous ore to a pretreatment step to deactivate or remove the preg-robbing component of the ore.
A further object of the present invention is to permit the recovery of precious metals values from low grade precious metal refractory ore material, including material that has heretofore been considered waste.
Still further, the present invention has as a goal the recovery of precious metal values from refractory precious metal ore material, particularly such ore materials with low grade precious metal content, with improved economic and energy efficiency.